The invention relates to a sensor for determining the concentration of gas components in a gas mixture. Such a sensor is especially suitable for monitoring the operability of catalytic converters in the exhaust-gas decontaminating systems of internal combustion engines.
Internal combustion engines generate exhaust gas which contains, inter alia, carbon monoxide, nitrogen oxide as well as uncombusted or partially combusted hydrocarbons. Measurements of the oxygen content in exhaust gases carried out with conventional lambda probes do not by themselves always provide sufficient data as to the quality of the combustion of the fuel mixture. For different applications, it is especially important to be able to control the oxidizable gas components occurring with incomplete combustions. In addition to reducing the limit values of the exhaust-gas emissions, the monitoring of the catalytic converter in exhaust-gas systems with respect to its function, such as in on-board diagnostics, is gaining greatly in significance.
Here, sensors are increasingly being used which are based on the mixture potential principle. Such sensors are, for example, disclosed in U.S. Pat. No. 6,022,464 as well as in German patent publication 4,408,504. The measurement principle of such sensors requires a very precise control of the temperature of the measurement electrodes. For this purpose, a heater control is used which uses the resistance of the heating device, which is integrated into the solid electrolyte of the sensor, directly as a temperature signal. The heater device is, for example, the resistance of a platinum heating device.
It is an object of the invention to provide an improvement of the temperature control for the measuring electrodes of a gas sensor based on the mixture potential principle.
A systematic error arises in sensors of this kind in that the temperature measurement does not take place at the location of the measuring electrodes but spatially separate therefrom on the heater resistor. Accordingly, it is still another object of the invention to be able to correct especially this systematic error.
The sensor arrangement of the invention is for determining the concentration of a gas component in a gas mixture. The sensor arrangement includes: a sensor including: at least one measuring electrode exposed to the gas mixture and from which a measurement signal is tapped; a reference electrode disposed in spaced relationship to the measuring electrode; and, a heater device having a heater element; and, a circuit unit including means for generating and supplying heating power to the heater element in an amount required to reach a pregiven value of the heater resistance of the heating element; and, means for generating a corrective value for changing the measurement signal in dependence upon the heating power.
By changing the measurement signal by a corrective value, a significant improvement of the correlation between the measurement signal of the sensor and the concentration of the exhaust-gas component to be measured is achieved. The corrective value is dependent upon the heating power of the heating device which is required to arrive at a pregiven value of the heater resistance. The exhaust-gas component is measured for different measuring parameters and/or at different operating points of an engine whose gas mixture is detected.
According to an advantageous embodiment of the invention, the corrective value for changing the measuring signal can be determined in dependence upon the change of the heating power by a measurement during operation and can be stored in a characteristic field. In this way, the functional relationship of the corrective value can be determined exactly and can be used to correct the measurement signal. The functional relationship is dependent upon the temperature dependency of the sensor signal.
In another advantageous embodiment of the invention, the corrective value is directly proportional to the change of the heating power. In this case, a corrective value need only be determined by measurement.
The sensor arrangement for determining the concentration of a gas component in a gas mixture includes: a sensor including: at least one measuring electrode exposed to the gas mixture and from which a measurement signal is tapped; a reference electrode disposed in spaced relationship to the measuring electrode; and, a heater device having a heater element having a resistance having a pregiven desired value; and, a circuit unit including means for increasing the desired value of the resistance at an operating point of the sensor having an increased heating power requirement so that a temperature drop associated with the increased heating power requirement is compensated with the temperature drop being to the at least one measuring electrode.
In this embodiment, the temperature at the electrode is held constant instead of correcting the temperature influence on the signal. In this way, the systematic error can also be corrected. The systematic error occurs in such sensors in that the temperature measurement is not made at the location of the measuring electrodes but is made at the heater resistor separated spatially from the measuring electrodes. Here, it is advantageous to determine the desired value by measurements in different operating states of the sensor and to store the same in a characteristic field.
The desired value is determined in dependence upon the temperature drop to the measuring electrodes with the temperature drop being detected via measurement.